12B4 CCDA.pdf - Tube CAD Journal

GlassWare Audio Design
Unlike the Aikido circuit, which delivers a perfect platform for tube rolling, as vastly
different tubes can be swapped in and out of the board (6AQ8 or 6H30) without
having to change the resistor values, the 12B4 CCDA requires more care in selecting
resistor values. The problem is the daunting array of different possible B+ voltages
and idle currents. For example, a 12B4 CCDA might run a B+ voltage of only 100Vdc
or as much as 300Vdc. Moreover, the plate resistor cannot be the little 1/2W devices
that the Aikido freely uses, but big 2W (or 3W) power resistors, which are hard to
find and expensive. The solution the problem of too many resistor combinations is to
let the idle current move, but lock the plate and cathode resistor values. A triode with
a cathode and plate resistors acts like a resistor, not a perfect resistor, but a fairly
good one.
As the graph above reveals, a 12B4 triode with a 1000-ohm cathode resistor (no plate
resistor) behaves much like a 9.4k resistor. (By the way, note the much improved
linearity over the plate curve lines, albeit at the cost of greatly increased plate
resistance and reduced transconductance.) Adding a plate resistor also makes the
triode behave more like a good resistor. The formula for the effective resistance is:
R = rp + Ra +(mu + 1)Rk.
The upshot is that if we chose plate and cathode resistors values to work at the
middle of possible B+ voltages, these same resistors will still split the B+ voltage
across a wide range of B+ voltages. In other words, we can use a wide range of B+
voltage with the same cathode and plate resistor values.
In general, the following formula is a good starting point:
Rk = (Ra – rp) / (mu + 1)